Materials such as glass materials, resin materials, single crystals and translucent ceramics have been conventionally used for lenses which are used as optical components for cameras and the like. In digital video cameras and digital still cameras applications, it is necessary to miniaturize optical systems, and it is desired to reduce the size of the lens itself and/or the number of lenses.
As possible means for reducing the number of lenses, the lenses are formed in an aspherical shape. This is because aspherical lenses are capable of correcting various aberrations, such as spherical aberration, while making use of the characteristic values of the materials of the lenses in comparison to spherical lenses. Furthermore, the size of a lens can be reduced by increasing the refractive index of the lens material.
In order to form an aspherical lens, a translucent resin film in an aspherical shape may be laminated on and bonded to a spherical lens serving as a lens base material using a glass material or translucent ceramic. This method is capable of forming an aspherical lens even when a lens base material cannot be spherically processed by molding. The resulting lens is referred to as a “hybrid lens”.
A general method for forming an aspherical hybrid lens is as follows. A predetermined amount of ultraviolet curable resin is poured into an aspherical mold provided with a transfer layer corresponding to a desired aspherical shape, and a surface of a lens base material which is to be formed in an aspherical shape is fixed to the mold so as to face the mold surface while keeping a constant distance between the surface and the mold. Then, the lens base material is irradiated with ultraviolet light to cure the resin. In this case, the resin is irradiated with ultraviolet light transmitted through the lens base material. Patent Document 1 discloses a composite aspherical lens as a conventional example in which an aspherical ultraviolet cured resin film is laminated on a spherical lens composed of a glass material.
As a resin other than the ultraviolet curable resin which can be used, a thermosetting resin has been employed. In this case, however, there is the problem that the formed aspherical shape has relatively low dimensional accuracy. In addition, it is necessary to precisely control heating and cooling conditions, and thus the thermosetting resin method is undesirable from the viewpoint of mass productivity. Therefore, a ultraviolet curable resin is generally used.
If the refractive indexes of the lens base material and the ultraviolet curable resin can be increased, the hybrid lens can be miniaturized to promote the miniaturization of an optical system. Patent Document 2 discloses translucent ceramic composed of a Ba{Sn, Zr (Mg, Ta)}O3 perovskite-type oxide having a refractive index of as high as 2.0 or more. In this specification, the term “refractive index” represents the refractive index at the visible light d-line (wavelength of 587.56 nm) unless otherwise specified.
However, lens materials composed of glass, which is typified in Patent Document 1, generally has a low refractive index of less than 2 and are thus disadvantageous for miniaturizing lenses. Although there are glass materials having a high refractive index of about 2.00, these materials have the problem of low ultraviolet transmittance when used as lens base materials.
More particularly, when the ultraviolet curable resin is irradiated with ultraviolet light, the intensity of the ultraviolet light applied to the resin varies according to site irradiated because the thickness of the lens material varies according to site, thereby causing a site dependency for the degree of resin curing. Therefore, the cured resin has a lack of homogeneity, and the optical properties of a hybrid lens as a completed product, i.e., the refractive index and transmittance, undesirably have site dependency. In addition, the lens may become cracked due to the occurrence of nonuniform stress.
On the other hand, if a lens base material has sufficiently high transmittance for ultraviolet light, even when the thickness of the lens material somewhat varies according to site, resin curing is little affected.
Patent Document 1: Japanese Examined Patent Application Publication No. 6-93043
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-75512